1. Field of the Invention
The present invention relates to optical scanning devices for use in image forming apparatuses.
2. Description of the Related Art
Image forming apparatuses that perform electrophotographic image recording by using lasers are widely used. Such an image forming apparatus generally includes an optical scanning device that outputs a laser beam and optically scans a surface of a rotating photosensitive drum in an axial direction with the laser beam so as to form a latent image on the photosensitive drum. The laser beam output from the optical scanning device is typically deflected by a deflector toward the photosensitive drum.
For example, Japanese Patent Application Laid-open No. H10-3048 discloses an optical scanning device for use in a multi-color image forming apparatus. The optical scanning device includes a light emitting unit, a deflection scanning unit, an image forming unit, a polarization splitting unit, and a deflecting unit. The light emitting unit emits a superimposed laser beam in which two laser beams with polarization directions differing by 90 degrees are superimposed. The deflection scanning unit performs deflection scanning in which the superimposed laser beam emitted from the light emitting unit is deflected and scanned. The image forming unit focuses the superimposed laser beam that is subjected to the deflection scanning by the deflection scanning unit. The polarization splitting unit splits the superimposed laser beam into two laser beams by making the superimposed laser beam to be reflected and transmitted. One of the laser beams, i.e. the reflected laser beam, is directed to the surface of a first photosensitive drum, and the other one of the laser beams, i.e., the laser beam passing through the polarization splitting unit, is directed toward the surface of a second photosensitive drum.
Japanese Patent Application Laid-open No. 2007-225680 discloses a scanning optical device and a color image forming apparatus that includes four photosensitive elements. The scanning optical device includes four semiconductor lasers, two rotating polygon mirrors, and four sets of scanning lenses.
Japanese Patent No. 3214944 discloses an optical scanning device. The optical scanning device is configured such that, a light beam emitted from a light source is deflected by a deflecting unit, passes through a half mirror that is inclined with respect to a plane of the deflected laser beam, and enters a reflective image forming element. The light beam reflected by the reflective image forming element is reflected by the half mirror and focused on a target surface as a light spot. In this manner, a reflective image forming element employed to perform image formation.
In recent years, there have been increasing demands for a thinner image forming apparatus. However, in the optical scanning device disclosed in Japanese Patent Application Laid-open No. H10-3048, a deflection surface at, the deflection scanning unit is not parallel to a surface including an irradiation position of each of the photosensitive drum, whereby the image forming apparatus including the optical scanning device becomes large.
In the color image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2007-225680, to obtain an optical path length corresponding to a length of the main-scanning direction, a certain distance needs to be secured between the rotating polygon mirror and the photosensitive drum, which causes a gap between the scanning optical device and the photosensitive drum to become large. The gap between the scanning optical device and the photosensitive drum could be narrowed by employing a plurality of reflecting mirrors; however, in that case the scanning optical device becomes large in the thickness direction in such a case.
In the optical scanning device disclosed in Japanese Patent No. 3214944, because the intensity of the laser beam irradiating the target surface decreases to ¼ when the laser beam passes through the half mirror or is reflected by the half mirror, a high-power light source is required, which increases manufacturing costs.